1. Field of the Invention
There are numerous commercial jet aircraft of various sizes including the large "jumbo jet" aircraft, such as the Boeing 747, McDonnell Douglas MD11 and Lockheed L1011. In still larger aircraft, such as the 600 passenger planes envisioned for the future, the loads on wing members needed to hold these aircraft aloft are somewhat heightened. Such large aircraft will carry in the neighborhood of 600 passengers and may include two passenger decks. While a Boeing 747 (one of the largest planes in commercial use) has an empty weight of about 399,000 pounds, it is estimated that future high capacity crafts may weigh as much as 532,000 pounds empty and somewhere around 1,200,000 pounds loaded. As used herein, the term "high capacity aircraft" refers to planes weighing more than 450,000 pounds empty. To heighten the overall efficiency of such aircrafts, it would be important to have materials in the wing structures that can support the load of these airplanes without themselves becoming too heavy. Aluminum alloys have seen wide use in airplane structural members, including airplane wing structural members, and have an enviable record for dependability and performance. More exotic, composite or other materials can be used for airplane wing structural members, but are much more costly and can be somewhat less dependable than aluminum alloys.
2. Technology Review
In general, the structural core of a large airplane wing typically includes a box-like structure made of an upper wing skin, lower wing skin, and end pieces for closing in the ends of this box-like structure. While the upper and lower members are labeled "skin", it is important to appreciate that these are not thin skins such as on the airplane fuselage, but rather thick plate products, for instance one half inch or more. In most of the current commercial jet aircraft, the upper wing skin is made of a 7000 Series alloy, currently a 7X50 alloy (As used herein, "7X50" refers to both 7050 and 7150 aluminum), or the more recently developed aluminum alloy 7055. U.S. Pat. No. 3,881,966 describes 7X50 alloys and U.S. Reissue Pat. No. 34,008 describes the use of 7150 aluminum as upper wing skins on a commercial jet plane. U.S. Pat. No. 5,221,377 describes alloy 7055 and refers to its use in aerospace structural members. Upper wing skins are normally artificially aged to T6-type or possibly T7-type tempers. U.S. Pat. Nos. 4,863,528, 4,832,758, 4,477,292, and 5,108,520 each describe 7000 Series aluminum alloy temperings which can be used to improve the performance of such alloys. All the aforesaid patents (U.S. Pat. Nos. 3,881,966, Re. 34,008, 5,221,377, 4,863,528, 4,832,758, 4,477,292 and 5,108,520) are fully incorporated herein by reference.
In commercial jet aircraft, the lower wing skins have generally been made of aluminum alloy 2024, or similar products such as alloy 2324 which is described in U.S. Pat. No. 4,294,625. The temper normally applied to these 2000 Series alloys is a T3-type, such as T351 or T39. All temper and alloy designations used herein are generally described in the Aluminum Association Standards and Data book, the pertinent disclosures of which are incorporated by reference herein.
Both the upper and lower wing skins of these aerospace box-like structures may be reinforced by stringer members having a channel, T- or J-type cross-sectional. Such stringer members are typically riveted to the inner surfaces of a wing skin to stiffen that skin and further stiffen the overall wing box structure. In general, when a commercial jet aircraft is in flight, the upper wing skin of this box is in compression and lower wing skin in tension. An exception occurs when the airplane is on the ground. There, the stresses are reversed but at much lower levels since the wing outboard of a landing gear virtually holds its own weight while on the ground. Thus, the more critical applications exist when an airplane is in flight to place the upper wing skin in compression and lower wing skin in tension.
There have been limited exceptions to the alloy selections for commercial jet planes described above. These include the Lockheed L1011 which used 7075-T76 lower wing skins and stringers and the military KC135 fueler plane which included 7178-T6 lower wing skins and stringers. Another military plane, the C5A, used 7075-T6 lower wing skins that were integrally stiffened by machined out metal. Military fighter planes such as the F4, F5E, F8, F16 and F18 have included lower wing materials of 7075 alloy or related 7475 alloy (F16 and F18). But, generally speaking, wing box structures for commercial jets over the years have included a 7000 Series alloy upper wing skin and lower wing skin made of a 2000 Series alloy, namely, 2024 aluminum or other member of the 2024 family.
The important desired properties for a lower wing skin in high capacity aircraft include higher strength, better fatigue life and improved fracture toughness, especially when compared to today's 2X24 equivalents. Current alloys for lower wing skin members in commercial jet aircraft all lack in the property needs required for tomorrow's high capacity aircraft.